Load Testing Batteries/How low can they go?

[uncle_bob]

Hi

Regardless of calling it a discharge test or calling it an accident, taking your batteries down to 10.5V is not good for them. If they are well made, doing it once or twice isn't going to ruin them. The "risk" of setting up fancy testing in your garage is that you do way more tests than you should.

If you do a DIY setup for a discharge test, be very careful about a couple of things:

1) You want it to fully cut off at whatever the manufacturer spec's as the magic end of capacity voltage. No parasitics, no leakage, shut off now.

2) You need to be reasonably sure that the device providing the load provides a constant load. That may not be easy to do. Most gear (inverters etc ...) change a bit as the voltage drops. (one solution is to use a shunt based battery monitor as part of the setup).

3) Battery capacity varies and the spec'd voltages vary over temperature. If you are testing out in the back yard, you need to compensate for this. Since it's a long test, the temperature may not be constant throughout the entire test. Indeed low temperature also gets into problems with not being able to properly change the battery ....

4) This is not a test that likely to destroy your trailer. There are risks, especially with a DIY setup. Keeping an eye on what's going on is highly recommended .....

So some math:

If the current varies by 10%, the result will likely be accurate to < 10%.

If the battery temperature is at 24 F (like my thermometer shows right now). 10.5V could easily be 11.1V. Full charge voltage at 24F is well above anything the stock charger will hit.

If you are after a 10A 12V load, number 14 wire will likely handle the load. You are after
1.2 ohms. The wire is 2.525 ohms per 1,000 feet. A 500 foot piece will get you pretty close .....Lay it out on the ground ( not in a big coil) and it should be very safe.


Fun !!!

Bob

[ROBERT CROSS]

[QUOTE=bls72bmw;2474153]These look reasonable and not crazy expensive.
Any feedback? (though the posts are a bit offset from the OEM batteries_)

https://www.oreillyauto.com/detail/c...j/34magm?pos=0

I wouldn't consider a 55ah 'starting' battery for an application without a starting requirement.

I always recharged our Lifeline AGM's at 12.2v, they lasted 11 Seasons in the AS and are still being used as back-up on the basement sump pump.

Bob
🇺🇸

[idoco]

*Addendum to my previous post on battery testing

If you have a battery monitor with a shunt you can forgo the clock and just read the amps used before shutoff directly from the monitor.

If you use a constant load (heater, light bulbs, etc) you need to switch off the breakers/fuses for all the 12V appliances that may be adding to the load.

In place of using a constant load (light bulbs, etc) you could just unplug your trailer and let it sit till the inverter shuts off at 10.5V. If your fridge is 12V leave it on. Maybe turn on your Alde so the circulator pumps run. And a light or two.

This will give you more of a "real life" capacity. Basically 50% of whatever the total amp hour usage on your monitor displays.

[uncle_bob]

Originally Posted by idoco

*Addendum to my previous post on battery testing

If you have a battery monitor with a shunt you can forgo the clock and just read the amps used before shutoff directly from the monitor.

If you use a constant load (heater, light bulbs, etc) you need to switch off the breakers/fuses for all the 12V appliances that may be adding to the load.

In place of using a constant load (light bulbs, etc) you could just unplug your trailer and let it sit till the inverter shuts off at 10.5V. If your fridge is 12V leave it on. Maybe turn on your Alde so the circulator pumps run. And a light or two.

This will give you more of a "real life" capacity. Basically 50% of whatever the total amp hour usage on your monitor displays.

Hi

The advantage of doing it with a monitor to the 50% point is that you have no heavy duty impact on the batteries life. The disadvantage is that you are depending on a "middle of the chart" voltage. A few tenths of a volt will make a big change in the outcome of the test. With the "normal" 10.5 V cutoff, a few tenths of a volt will have a fairly minor impact.

If you take the monitor to the 10.5V level, make sure that it will still operate at that voltage. Some will, some will not.

Inverters are rated to "cut out below 10.5V", at least according to the fine print on some of them. That's not quite the same as "cut out at 10.5V". There are some that with small loads, will quite happily keep running down to 7 or 8 volts. You don't want your battery to go that low .....

Bob

[Bill M.]

So capacity testing a marginal battery is sorta like dunking a witch to see how she does?

[uncle_bob]

Originally Posted by Bill M.

So capacity testing a marginal battery is sorta like dunking a witch to see how she does?

Hi

A little bit .... the first dunk likely won't do massive damage. Keep doing it again and again ... there will be nasty side effects.

Bob

[idoco]

Originally Posted by uncle_bob

Hi

The advantage of doing it with a monitor to the 50% point is that you have no heavy duty impact on the batteries life. The disadvantage is that you are depending on a "middle of the chart" voltage. A few tenths of a volt will make a big change in the outcome of the test. With the "normal" 10.5 V cutoff, a few tenths of a volt will have a fairly minor impact.

Agreed. The problem, as you said, is how you define 50%.

If it 50% SOC open circuit voltage. Which means you have to periodically shed the load and check the voltage. Also what is "really 50% SOC" open circuit voltage.

Is it 50% SOC under load circuit voltage. In which the "50%" voltage may be different for the same SOC under different loads.

Is it drawing 50% of the rated AH capacity of the battery. In which case if your battery has lost capacity it could be 50% or could be 90%! (FWIW I think the Victron monitors display the percent remaining as a function of the AH removed. Based on the rated AH capacity entered into the monitor.)

My point being that whatever method used is a "squishy estimate". But 1.75V/cell (10.5V per 12V battery) is the "least squishy". As long as you know the inverter shutoff point or can set it, I think it's relatively safe for your batteries. Our Xantrex inverter automatic shut off can be set for between 10.5V-12.1V

[uncle_bob]

Originally Posted by idoco

Agreed. The problem, as you said, is how you define 50%.

If it 50% SOC open circuit voltage. Which means you have to periodically shed the load and check the voltage. Also what is "really 50% SOC" open circuit voltage.

Is it 50% SOC under load circuit voltage. In which the "50%" voltage may be different for the same SOC under different loads.

Is it drawing 50% of the rated AH capacity of the battery. In which case if your battery has lost capacity it could be 50% or could be 90%! (FWIW I think the Victron monitors display the percent remaining as a function of the AH removed. Based on the rated AH capacity entered into the monitor.)

My point being that whatever method used is a "squishy estimate". But 1.75V/cell (10.5V per 12V battery) is the "least squishy". As long as you know the inverter shutoff point or can set it, I think it's relatively safe for your batteries. Our Xantrex inverter automatic shut off can be set for between 10.5V-12.1V

Hi

It's way worse that that

All the magic voltages change with temperature so you need to know the temperature inside the battery. Also the voltages change with the battery plate composition. Turns out the manufacturers optimize for different regions. Alaska gets a different plate than Arizona (at least from some outfits). All of this really messes up a "mid point" measure.

Indeed it messes up any measurement, but the battery is dead anywhere near 10.5V ....

Bob

[mythbuster88]

Originally Posted by bls72bmw

These look reasonable and not crazy expensive.
Any feedback? (though the posts are a bit offset from the OEM batteries_)

https://www.oreillyauto.com/detail/c...j/34magm?pos=0



Going to do a 'test use' of the batteries as one of the above gents suggested...use the trailer in the backyard for a weekend and see how the batteries do in real-ish world conditions. Also as a way to get used to the camper.

I think it's a lot of overthinking we all do on this stuff...I'm in the middle of a city and will do a demo, if they work, great, if not, get new ones. LOL.
Too much brain space can be dedicated to the Rabbit Hole where-as I have a million other things to get on with.
Of course, to each their own, that's what having a nice discussion here does! Thanks for the input.

Great idea to test in the backyard.
While at it test and run everything else.
For us the critical battery test is will we be able to run the forced air heater and also watch a few movies over a weekend.

[Bill M.]

There is no way you are going to be able to run the furnace "over the weekend" on batteries if it is very cold or if the furnace runs much. Even on fresh batteries. You need to look at your usage. The furnace is a big draw.

[mythbuster88]

Originally Posted by Bill M.

There is no way you are going to be able to run the furnace "over the weekend" on batteries if it is very cold or if the furnace runs much. Even on fresh batteries. You need to look at your usage. The furnace is a big draw.

That is why we tossed the lead acid batteries in the trash and went to lithium.

[Bill M.]

I mis read your post. Thinking about the lithium upgrade myself.

[jrmillerjr]

Does anyone have experience with the Noco Genius smart chargers? Sounds like they analyze then charge according to the battery state. I have 2 Lifeline GPL-24T in series that I may have damaged. The trailer was completely dead after a cold overnight. I was able to recharge off shore power but am now rarely seeing 100% with quicker discharge, often below 50%. Is it worth $70 to see if they can restore the batteries or should I just bite the bullet and upgrade to lithium?

[dznf0g]

Originally Posted by jrmillerjr

Does anyone have experience with the Noco Genius smart chargers? Sounds like they analyze then charge according to the battery state. I have 2 Lifeline GPL-24T in series that I may have damaged. The trailer was completely dead after a cold overnight. I was able to recharge off shore power but am now rarely seeing 100% with quicker discharge, often below 50%. Is it worth $70 to see if they can restore the batteries or should I just bite the bullet and upgrade to lithium?

I am not a fan of Noco, based on my son's experience with one which was OEM on his trailer. Failed very early on in its life.

[uncle_bob]

Originally Posted by jrmillerjr

Does anyone have experience with the Noco Genius smart chargers? Sounds like they analyze then charge according to the battery state. I have 2 Lifeline GPL-24T in series that I may have damaged. The trailer was completely dead after a cold overnight. I was able to recharge off shore power but am now rarely seeing 100% with quicker discharge, often below 50%. Is it worth $70 to see if they can restore the batteries or should I just bite the bullet and upgrade to lithium?

Hi

If anything, get a shunt based battery monitor:

https://shop.pkys.com/victron-energy...y-monitor.html

Is one of many. Peter is a good guy and his shop isn't that from from you.

Trying to work out battery state from voltage is impossible if you don't know the battery temperature *and* have the correction factors for your battery. It's the cold season so all the "magic voltages" will be higher than you think.

Your stock charger (and most replacement units) does not monitor temperature. It's confused about charge state as well. That results in the battery not fully charging, even after being on charge for quite a while. Add the fact that battery capacity decreases when it's cold and you have a double whammy.

Best guess is your batteries are ok. Take them in to a good shop and have them checked. If the fail (and are not replaced under warranty), *then* start thinking about lithium .....

Wacky converters are *not* a good idea in an RV. Some of them put out crazy high voltages. Blowing out appliances on the trailer is not any fun. The standard designs are all pretty conservative for that reason ....

Bob

[Its4kito]

I installed my 1 year old AGM into my 2018 Basecamp. I also have three 100 W solar panels. The batteries were charged before winter storage in my basement. My solar control read 14.4 V after I installed them. After FOUR hours, the controller read 10.5 V. I replaced the factory Interstate batteries fir the same reason after learning one of them was bad.I’ve gone camping with this 2018 Basecamp and haven’t been able to rely on heater or fridge ( after faulty ammonia circulation repair) when the battery goes below 10.5.

[uncle_bob]

Originally Posted by Its4kito

I installed my 1 year old AGM into my 2018 Basecamp. I also have three 100 W solar panels. The batteries were charged before winter storage in my basement. My solar control read 14.4 V ....

Hi

Here's the problem, if the batteries were cold, they may have only been to 50% charge when the voltage read 14.4. You can not trust voltage readings to tell you state of charge.

Bob

[OTRA15]

Batteries are batteries . . . regardless of which model Airstream they are in.

FYI the main Batteries etc. forum has many possible relevant threads:

https://www.airforums.com/forums/f449/

These "load testing batteries" search results may also lead to productive pastures IMO:

https://www.google.com/search?q=load...=airforums.com

Good luck.

FYI
FWIW